Drilling offshore oil and gas wells includes the use of offshore platforms for the exploitation of undersea petroleum and natural gas deposits. In deep water applications, floating platforms (such as spars, tension leg platforms, extended draft platforms, dynamically positioned platforms, and semi-submersible platforms) are typically used. One type of offshore platform, a tension leg platform (“TLP”), is a vertically moored floating structure used for offshore oil and gas production. The TLP is permanently moored by groups of tethers, called tension legs, that eliminate virtually all vertical motion of the TLP. Another type of platform is a spar, which typically consists of a large-diameter, single vertical cylinder extending into the water and supporting a deck. Spars are moored to the seabed like TLPs, but whereas a TLP has vertical tension tethers, a spar has more conventional mooring lines.
Offshore platforms typically support risers that extend from one or more wellheads or structures on the seabed to the platform on the sea surface. The risers connect the subsea well with the platform to protect the fluid integrity of the well and to provide a fluid conduit between the platform and the wellbore.
Risers that connect the surface wellhead on the platform to the subsea wellhead can be thousands of feet long and extremely heavy. To prevent the risers from potentially buckling under their own weight or placing too much stress on the subsea wellhead, upward tension is applied, or the riser is lifted, to support a portion of the weight of the riser. Since offshore platforms often move due to wind, waves, and currents, for example, the risers are tensioned such that the platform can move relative to the risers. To that end, the tensioning mechanism often exerts a substantially continuous tension force on the riser.
Risers can be tensioned by using buoyancy devices that independently support the riser, which allows the platform to move up and down relative to the riser. This isolates the riser from the heave motion of the platform and eliminates any increased riser tension caused by the horizontal offset of the platform in response to the marine environment. This type of riser is referred to as a freestanding riser.
Hydro-pneumatic tensioner systems are another type of a riser tensioning mechanism. In this type of system, a plurality of active hydraulic cylinders with pneumatic accumulators is connected between the platform and the riser to provide and maintain the desired riser tension. The platform's displacement, which may be due to environmental conditions, that causes changes in riser length relative to the platform are compensated by the tensioning cylinders adjusting for the movement.
Floating platforms, which are used for deeper drilling and production, often encounter additional challenges, such as thermal expansion, due to the fact that the drilling extends into very high temperature formations where special drilling equipment may be required. At high temperatures, the riser, which extends from the sea floor, is subject to expansion and contraction. And that expansion and contraction of the production/drilling riser may result in undesirable movement, such as buckling, in response to temperature changes.